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Summary
Ordinary matter is made of fast-moving electrons and slow-moving nuclei. The dynamics of electrons accounts for most material properties from color to reactivity. Our success in predicting the physics of semiconductors and the chemistry of molecules depends on our ability to characterize the electron. The electron is vanishingly small, and its properties can be well described by three parameters: mass, charge, and magnetic dipole. All of these parameters have been measured to extreme precision. Theoretical models of physics predict that the electron will also have a very small electric dipole moment. A strong electric field is required to detect a small electric dipole, and one proposed method is to observe the properties of the electron in the internal electric field of a polar molecule. Loh et al. (1) recently demonstrated that molecular ions can also be used for this task. On page 269 of this issue, Baron et al. (2) report on measurements that lower the bound for the electron's electric dipole moment.